1. Field of the Invention
This invention relates to a predictive coding method and apparatus for use in the highly efficient coding of moving image signals, such as, television signals and the like.
2. Description of the Prior Art
A predictive coding system is known for effecting the efficient coding of moving image signals, such as, television signals, and the like. Such known predictive coding system, for example, as disclosed in "Digital Image Processing", by Rafael C. Gonzales and Paul Wintz, 1977, Addison-Wesley Publishing Co., Reading, Mass., at pages 278-279, includes encoding and decoding sections as generally shown in FIGS. 1 and 2, respectively, hereof.
The encoder is shown in FIG. 1 to comprise an input terminal 1, a subtractor 2, a requantizer 3, an adder 4, a predictive filter or predictor 5 and an output terminal 6. The terminal 1 receives a suitable digital input signal which is, for example, obtained by sampling a television signal, and which is applied to one input of the subtractor 2. Another input of the subtractor 2 receives the output of the predictive filter 5 which employs a predictive function or coefficient P(Z) for providing a predictive or current value from a comparative value or previous input digital data, for example, the next preceding sample. The subtractor 2 provides, at its output, a residual signal, that is, the difference between the value of the input signal supplied thereto from the input terminal 1 and the predictive value from filter 5. The residual signal from the subtractor 2 is supplied to the requantizer 3, wherein the residual signal is quantized and delivered to the output terminal 6. As the predicting accuracy of the filter 5 is increased, that is, as the predictive value from the output of the filter 5 becomes nearer to the value of the input signal from the terminal 1, the value of the residual signal is correspondly reduced and, accordingly, the information of output terminal 6 can be further compressed.
The quantized residual signal is also shown to be supplied to the adder 4 for adding to the predictive value from the predictive filter 5. The output from the adder 4, that is, the sum of the quantized residual signal or value and the predictive value, is supplied to the predictive filter 5 as a comparative value to be used in the prediction of the next input data sample.
The decoder employed in the known predictive coding system is shown in FIG. 2 to comprise an input terminal 7, an adder 8, a predictive filter or predictor 9 and an output terminal 10. The adder 8 adds a predictive value from the predictor 9 to the data of the quantized residual signal suitably transmitted from the output terminal 6 to the input terminal 7 to thereby decode the original sample data and deliver the same to the output terminal 10. The predictive filter or predictor 9 is similar to the predictor 5 of the encoder in that it employs a predictive function P(Z) to predict a current value from a comparative value or a previous input signal, for example, the decoded output from the adder 8 which corresponds to the next preceding sample.
The above-described known coding system, if it is applied to the coding of a television signal representing a moving image, transmits only those portions of the signal representing the movement and accordingly can effect a highly efficient coding of the television signal. By transmitting only a difference signal, that is, the difference between each input signal and the corresponding predictive value, the word length can be compressed. Further, the compression of the information is made possible by providing entropy encoding and decoding corresponding to the probability density distribution of the difference signal.
It is desirable, in the described predictive coding system, to improve the accuracy in the predictive function P(z) and to minimize the addition of squared residual signals for compressing the word length as well as for effectively applying the entropy or variable length coding. However, when the above described coding system is applied to an ordinary broadcasting television signal or the like, if only a single predictive function or coefficient is provided, it cannot cope with various statistical characteristics of the input signal which substantially vary with the lapse of time, so that the accuracy in the predictive function may be insufficient.
Thus, a system has been proposed in which the characteristics of a predictive function are varied in accordance with the statistical characteristics of an input signal. Further, a system has been proposed in which one of a plurality of different predictive functions is selectively used in accordance with the statistical characteristics of the input signal.
The systems which selectively employ a plurality of different predictive functions can be roughly classified into systems of the following two types:
(1) A system which selects an optimal predictive function for each pixel or each pixel block and which transmits the selection control information for the optimal predictive function along with the data indicative of the difference between an input signal and a predictive value; and
(2) A system in which an encoder and a decoder each select an optimal predictive function without requiring the transmission of the selection control information.
In the case of a system of the type indicated at (1) above, if the selection control information for the optimal predictive function is transmitted for each pixel, an optimal predictive function can be selected for each pixel and the addition of squared residual signals is reduced to a minimum. However, the amount of transmitted information is increased by the amount of the transmitted selection control information. The amount of the selection control information to be transmitted may be decreased by selecting an optimal predictive function for each of a plurality of blocks each formed of a plurality of pixels. However, a predictive function thus selected for one block will not always be optimal for all of the individual pixels therein. In other words, if the block is made larger so that the amount of the transmitted selection control information is decreased, the amount of the residual signal that needs to be transmitted is increased. On the contrary, if the block is made smaller, the amount of the residual signal to be transmitted is decreased, but the amount of the selection control information to be transmitted is increased. Thus, problems remain whether the block size is made larger or smaller. Although it may be advantageous to make the block size variable, it would be difficult to devise the hardware for implementing such concept.
In the case of a system of the type indicated at (2) above, if it is possible to select an optimal or nearly optimal predictive function for each pixel in the encoder and the decoder of the system, the above-mentioned problems of the type (1) system can be solved. However, in the type (2) systems, it is difficult to derive the algorithm for selecting an optimal predictive function from a plurality of predictive functions.
In a proposed example of such selection algorithm for each of the encoder and decoder, an intermediate one of the amplitudes or values predicted by a plurality of predictive functions is selected, for example, as disclosed specifically in Telecommunication Technology, 1E83-83 published by Japan Institute of Electronics and Telecommunications. Although such algorithm can avoid the selection of the worst predictive value, the probability of selecting the best predictive value may be reduced for certain contents of images.
Further, even though the number of possible predictive functions is increased to improve the accuracy thereof, the advantages thereof may not be as pronounced as expected, since the described algorithm merely selects an intermediate value. In other words, the accuracy will not be improved in proportion to the increased number of the predictive functions from which the selection is made. Further, increasing the number of the predictive functions also results in undesirable complexities in the circuit arrangement for selecting the intermediate one of the predictive values. Moreover, this type of previously proposed predictive coding system carries out many operations for processing each pixel s that the hardware therefor has to have a high processing speed which is disadvantageous when the entire apparatus is to be integrated in a single chip.